73Shark is correct. Before it was an Interstate, it was a Sprinter cargo van, and it's always going to ride like a cargo van, unless you trade out the cargo van suspension for an air suspension.

One thing that some have noticed is, the more weight you have in the rear— up to the gross rear axle weight, of course— the better the ride quality in the rear seats. Which makes sense, because that's the weight the stock rear suspension was designed for. So make a point of storing your heaviest gear as far to the rear as you can.

Another tip that I've seen in several places is to match your tire inflation to the load. Apparently, tire vendors have documentation that indicates how much pressure you should use depending on how much weight the tire is carrying. Tires aren't as important as springs, but they say it does help.